In manufacture of a semiconductor laser, the wafer is cleaved into a plurality of bars by application of appropriate pressure after forming scribe marks on the wafer. However, it has been difficult to cleave a GaN wafer, since GaN crystal is highly stable.
To address this problem, Japanese Patent Laid-Open Publication No. 10-242570 (1998) discloses a wafer cleaving method, which includes forming on the wafer an auxiliary cleavage region layer having notches and cleaving the wafer along these notches. Further, Japanese Patent Laid-Open Publication No. 2002-64237 discloses another wafer cleaving method, which includes forming in the wafer a primary guide trench and a secondary guide trench having a smaller width than the primary guide trench such that the primary guide trench extends from an edge of the wafer to its center portion and these guide trenches are coupled together with their central axes coincident, and then cleaving the wafer along the direction in which these trenches extend.
However, the wafer cleaving methods of the former publications are disadvantageous in that the methods require re-growing a predetermined semiconductor layer region to form the auxiliary cleavage region layer. Further, it is difficult to accurately cleave a GaN wafer in a desired plane by the method of the latter publication, which forms guide trenches only at an edge of the wafer. The reason for that is that a GaN wafer tends to cleave in directions at an angle of 60 degrees with respect to a desired cleavage direction since GaN crystal is in a hexagonal system.
Another conventional method for accurately cleaving a GaN wafer in a desired plane has been to form scribe marks on a per-chip basis instead of forming the scribe marks just at an edge of the wafer. However, if these scribe marks are misaligned, that is, if one of the two facing scribe marks is not on the desired cleavage plane, cleaving the wafer using those scribe marks may result in a cleaved surface having a step-like portion. Such a step-like portion tends to be formed at a position intermediate between the two scribe marks, especially at the ridge portion of the semiconductor laser since the ridge portion is likely to suffer stress concentration. This may result in degraded characteristics of the semiconductor laser since the ridge portion includes the light-emitting region.
FIG. 12 is a schematic cross-sectional view of a ridge portion 101 having a step-like portion, or streaks, formed due to cleavage. This step-like portion is herein referred to as a defective portion 102. In FIG. 12, the defective portion 102 extends into that part 103 of the ridge portion 101 through which the emitted light widens, which adversely affects the characteristics of the light.
The present invention has been devised in view of the above problems. It is, therefore, an object of the present invention to provide a method for manufacture of a semiconductor laser that allows cleaving the wafer in a desired plane.
Another object of the present invention is to provide a method for manufacture of a semiconductor laser that prevents the cleaved surface from having a step-like portion.
Other objects and advantages of the present invention will become apparent from the following description.